1. Field of the Discovery
The field of the presently disclosed apparatus relates to filling a void's volume with a conformal heater. More specifically the apparatus relates to the use of conductive linings or imprints surrounding a conformal core to fill the void's volume and/or impart heat energy to a void's walls through more efficient conduction and heat transfer. The conformal core will need to impart force that maintains the surface contact between the heater and material wall.
2. Description of Related Art
The terms “elastic” and “elastomeric” are used herein to mean any material which, upon application of a biasing force, is stretchable, to a stretched, biased length which is at least about ten percent (10%) longer than its relaxed unbiased length, and which, will recover at least fifty percent (50%) of its elongation upon release of the stretching, elongating force. A hypothetical example would be a one inch sample of a material which is stretched to at least 1.10 inches and which, upon being elongated to 1.10 inches and released, will recover to a length of not more than 1.05 inches. Many elastic materials may be elongated by much more than ten percent (10%) (i.e., much more than one hundred ten percent (110%) of their relaxed length), for example, elongated by two hundred percent (200%) or more, and many of these will recover to substantially their initial relaxed length, for example, to within one hundred one percent (101%) of their initial relaxed length, upon release of the stretching force.
The term composite is commonly used in the industry to identify components produced by impregnating and/or encapsulation a filler material, often comprised of particles or fibers, with a resin material. Examples of fillers include, but are not limited to cloth fabrics, carbon nanotubes and carbon nanofibers, metallic fabrics and particles, wood fibers and particles, and other similar fabrics, fibers, and particles. Generally, polymers and polymer composites have the advantages of weight saving, high specific mechanical properties, and good corrosion resistance which make them indispensable materials in all areas of manufacturing. Nevertheless, manufacturing costs are sometimes detrimental, since they can represent a considerable part of the total costs and are made even more costly by the inability to quickly and easily repair these materials without requiring a complete, and expensive, total replacement. Furthermore, the production of complex shaped parts is still a challenge for the composite industry. The limited potential for complex shape forming offered by advanced composite materials leaves little scope for design freedom in order to improve mechanical performance and/or integrate supplementary functions. This has been one of the primary limitations for a wider use of advanced composites in cost-sensitive large volume applications.
Shape memory polymers (SMPs) and shape memory alloys (SMAS) were first developed about twenty (20) years ago and have been the subject of commercial development in the last ten (10) years. SMPs are polymers that derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation.
SMPs have a dynamic modulus that undergoes a sharp change in the modulus of elasticity at its glass transition temperature (Tg). This sharp change facilitates easy molding and forming of SMPs or composites wherein the resin used in the composite is an SMP. SMPs which have been preformed can be deformed to any desired shape below or above its Tg. If the SMP is below the Tg, this process is called cold deformation. When deformation of the SMP occurs above its Tg, the process is denoted as warm deformation. In either case the SMP must remain below, or be quenched to below, the Tg while maintained in the desired deformed shape to “lock” in the deformation. Once the deformation is locked in, the polymer network cannot return to a relaxed state due to thermal barriers. The SMP will hold its deformed shape indefinitely until it is heated above its Tg, whereupon the strain energy, stored as potential energy in the SMP, is released and the SMP returns to its preformed state. Typically, SMPs are deformed above their Tg because of the ease of deforming the SMP at these temperatures versus deforming the SMP at temperatures below their Tg. Additionally, SMPs have can have a higher strain imparted on the SMP before failure if deformed above their Tg.
SMPs are not simply elastomers nor simply plastics. They exhibit characteristics of both materials, depending on their temperature. While rigid, and below its Tg, an SMP demonstrates the strength-to-weight ratio of a rigid polymer; however, normal rigid polymers under thermal stimulus simply flow or melt into a random new shape, and they have no “memorized” shape to which they can return. While heated and pliable, above its Tg, an SMP has the flexibility of a high-quality, dynamic elastomer, typically tolerating four hundred percent (400%) elongation or more; however, unlike normal elastomers, an SMP can be reshaped or returned quickly to its memorized shape and subsequently cooled into a rigid plastic. If deformed or reshaped above its Tg and this deformation is maintained while the SMP is cooled, the SMP will retain this new, deformed, shape while below its Tg. If the SMP is then reheated to above its Tg the SMP, without additional force, will return to its original memorized shape.
Several known polymer types exhibit shape memory properties. Probably the best known and best researched polymer types exhibiting shape memory polymer properties are polyurethane polymers. Other SMP polymers known in the art include articles formed of Norborne or dimethaneoctahydronapthalene homopolymers or copolymers, set forth in U.S. Pat. No. 4,831,094. Additionally, styrene copolymer based SMPs are disclosed in U.S. Pat. No. 6,759,481 and Shape Memory Cyanate Ester Copolymers are disclosed in PCT application WO/2005/108448 published Nov. 17, 2005, which are incorporated herein by reference.
The primary design components of thermally activated SMPs include at least one monomer, possibly a co-monomer, a crosslinker, and possibly an initiator and additional filler material. A polymer engineered with shape memory characteristics provide a unique set of material qualities and capabilities that enhance traits inherent in the polymer system. SMPs can be chemically formulated with a transition temperature to match most application needs. It can be cast and cured into an enormous variety of “memorized” shapes, from thick sheets and concave dishes to tiny parts or a complicated open honeycomb matrix.
Methods other than thermal energy can activate the shape memory properties of SMP. Electromagnetic radiation, UV light and magnetism can be used to activate the SMP. Throughout this application “activation” will be defined as transitioning the material from a hard, rigid state to a soft, pliable and elastic state. Additionally, throughout this application, “deactivation” will be defined as transitioning the material from a soft, pliable state to a hard, rigid state.
Sheets of conductive metalized fabrics are well known in the art. In one method for introducing metal into the fabric of a composite, metal threads are woven into the graphite fabric at regular intervals. While this prior art technique has been proven satisfactory for most cases, it is evident that due to the inability of a metal thread to stretch, any strain is likely to break some, if not all, of the metal threads, reducing the conductivity of the composite.
In a second prior art technique for introducing metal into a composite, each fiber of the outermost layer is coated with metal prior to being woven into a continuous sheet. This technique is particularly disadvantageous in that the coaxial metal sheath around each fiber has a substantially different modulus of elasticity than the fiber itself. Thus, when the composite is subject to bending moments, the metal sheath tends to shear away from the fiber. In addition, unnecessary excess weight is introduced into the fabric weave.
A third prior art technique for introducing metal into a composite is shown by EEONYX Corporation (www.eeonyx.com). They created a intrinsically conductive polymer (ICP) with a chemical composition of polyaniline or polypyrrole. Those ICPs can only be added to plastics with a lower melting point of up to two hundred degrees Celsius (200° C.), and are therefore very limited in their use. The ICP can be prepared and deposited into a carbon black or other matrix and increase the use of plastics with three hundred degree Celsius (300° C.) melting point. The use of ICPs improves the electrical, mechanical and melt flow properties and greatly reduces the compounding difficulties and easier end-product fabrication of composites. In certain applications, the plastic exhibits a ten-fold increase in conductivity compared to high structure carbon black loaded alloys at the same loading level.
U.S. Pat. No. 4,657,807 issued to Myron M. Fuerstman on Apr. 14, 1987, discloses a method of depositing metal onto fabrics such as cotton and polyester. The process used according to Fuerstman was to select a fabric capable of flattening or polishing under heat and pressure, pressing the fabric against a heated surface and then vacuum metalizing the fabric by vapor deposition.
U.S. Pat. No. 4,764,665 issued to Ralph Orban, et. al, on Aug. 16, 1988 discloses several uses of a metalized conductive fabric. Specifically, Orban discloses use of the metalized fabric as a resistive heating element for use on airplane wings and in clothing, specifically gloves. The gloves are electrically heated woven fabric in which the fabric has been coated with electrically conducting metal to enable its use as a heating element. The fabric is in the shape of a hand.
U.S. Pat. No. 5,089,325, issued on Feb. 18, 1992, and U.S. Pat. No. 4,892,626 issued on Jan. 9, 1990, to James Covey discloses methods for plating one side of a woven fabric sheet by using a backing layer applied to one side of the sheet. The sheet and backing layer are wetted in an electrolytic solution containing metallic ions to be deposited on one side of the fabric sheet only. Air bubbles trapped in interstices of the fabric weave and beneath the backing sheet prevent the electrolytic solution from soaking through the fabric sheet. Electrodes bond the metal ions on the wetted fabric thereto. The backing sheet is then removed. The resulting fabric is coated on only one side and the interstices are not filled in by plating material. The fabric is useful as the outermost layer in a composite laminate for an aircraft skin.
Additionally, U.S. Pat. No. 7,078,658 issued to Daniel Brunner and André Amari on Jul. 18, 2006 discloses a method for using conductive fibers as a heater mat for aircraft. The heater mat is provided with a resistive element including at least two substantially parallel segments of electrically conductive fibers disposed on the aerodynamic surface of an aircraft. The segments come from a single strip of electrically conductive fibers, with two adjacent segments being obtained by folding a portion of the single strip at least twice. However, similarly to the previously discussed methods, Brunner fails to disclose a method of creating conductive patterns on a piece of fabric for use in a composite and limits the composite to those containing carbon fibers.
The drawbacks of the present methods, including the imprinting of a design onto a fabric by pressing metal foil which is well known in the art, include the inability to selectively deposit the material onto a particular portion of the fabric. Orban, while showing the ability to cut a piece of conducting fabric into a predetermined shape, does not disclose or show a means for selectively depositing metal onto only a predetermined portion of a single side or both sides of a piece of fabric. Similarly Fuerstman does not disclose a means to selectively deposit metal in a predetermined shape or pattern onto a fabric. Finally, both patents issued to Covey only disclose a means for depositing metal onto a single side and does not disclose how to imprint or deposit metal onto a fabric in a predetermined shape or pattern.
The two principal methods of creating high-strain conductive elastomers focus on developing the elastomer at the nano scale or involve the addition of conductive nano-sized fillers including: carbon nano-fibers/tubes, carbon black, nickel nano-strands, nickel coated graphite particles, and other nano-sized conductors.
The first major effort to create conductive elastomers, polymers and polymer composites was through the use of fillers. These processes use nano-sized conductive additives to increase the conductivity of elastomers, polymers, and polymer composites. High percent loadings of fillers, ten percent (10%) or more, are required to achieve useful electrical conductivity, resulting in a large decrease in the maximum percent elongation and ultimate tensile strength of the base elastomer. The electrical resistance of these materials also increased significantly with percent strain, rendering circuit design with these materials essentially impossible. The use of small, nano-sized carbon tubes, or other conducting material, as filler, to create a conductive elastomer, polymer and polymer composite is well known in the prior art.
One example of the use of fillers to create a conductive elastomer involves tailoring the electrical conductivity in elastomeric and polymeric materials used to build military and commercial aerospace components, with negligible impact on the elastomer's mechanical properties or manufacturing ability. This technology transforms almost any common elastomer or polymer into a multifunctional material capable of carrying or dissipating a significant electrical charge, an advancement offering tremendous promise throughout the space, aerospace, automotive, and chemical industries. This is controlled by dispersion of specifically designed, highly electrically conductive, yet remarkably flexible, carbon nano-tubes into the supporting elastomer or polymer matrix. These nano-tubes have the current carrying capacity of copper but with a comparatively much lower density.
The nano-tubes used in the finished products are on the order of sixty to two hundred nanometers in diameter with an aspect ratio (the ratio of their length to their diameter) of greater than eight hundred. The electrical and thermal conductivity of these nano-tubes is highly dependent on the architecture and design of the nano-tubes. This high aspect ratio results in a much lower required filler content to achieve percolation (onset of conductivity) than traditional metal-filled systems. The percolation threshold for these materials is less than one half of one percent by volume. The multi-wall nano-tubes used in this process are available in ton quantities, which allow affordable, realistic scale-up of the resultant nano-composites. Other examples of use of nano-tubes of conducting material are well known in the art.
The principle disadvantages of this method are the difficulty in achieving and maintaining the proper alignment of nano-tubes in the resin/elastomer matrix to ensure good conductivity. The specific environmental conditions and special equipment needed make these production methods very expensive. Additionally, as described in Effect of Strain on the Properties of an Ethylene-Octene Elastomer with Conductive Carbon Fibers, L. Flandin, et. al., Journal of Applied Polymer Science, 2000; 76 (6): 894-905, 897; Practical Considerations for Loading Conductive Fillers into Shielding Elastomers, Brian W. Callen and James Mah, ITEM 2002, 130-137, 134; and Interrelationships Between Electrical And Mechanical Properties Of A Carbon Black-Filled Ethylene-Octene Elastomer, L. Flandin, A. Hiltner, E. Baer, Polymer, January 2001 827-838, 831; the resistance of these materials likely dramatically increases under strain and the maximum strain decreases as the percentage of nano-tubes increase.
Because of the loading requirements and the size of the tubes it would be nearly impossible to imprint electrical circuit designs onto this type of composite as the scale of the conductive elements is not easily manipulated.
The second major area of investigation involved the manufacture of conductive polymers on the nano-scale. The process typically involved manipulating molecules to achieve a desired set of material characteristics by allowing only some molecules to bond to particular sites.
One of the latest attempts at solving the problem on the nano-scale involves NanoSonic, Inc.'s process to produce what it calls Metal Rubber™ U.S. Pat. No. 6,316,084 issued Nov. 13, 2001 to Richard O. Claus and Yanjing Liu covers some of NanoSonic's technology. According to Claus and Liu the material can be stretched to about three hundred percent (300%) of its original length and relax back. It can be exposed to chemicals, boiled in water overnight, and it doesn't mechanically or chemically degrade. Additionally it can be heated to approximately three hundred seventy degrees Celsius (370° C.), and it maintains its properties.
Metal Rubber™ is made using a nanotechnology process call electrostatic molecular self-assembly which means that Metal Rubber™ is formed one layer at a time where individual molecules are formed layer by layer on a surface. Starting with a plastic or glass substrate, or base, that is given an electric charge, either positive or negative, the plate is dipped alternately into two water-based solutions, one containing plastic molecules that have been given a positive electrical charge and the other containing plastic molecules with a negative charge. If the base has a positive charge, it goes into the negative molecules first, and they cling to the base, forming a layer only one molecule thick. After the next dipping, into positive molecules, a second ultra thin layer forms and this process will continue until the completed product is formed.
The biggest challenge to accurate and mass production is that the process requires the layers to be built one molecule layer thick at a time, which is very time consuming. Another difficulty is that the fabric size capable of being produced is limited to the plate size and is not readily capable of mass production. The added expense of chemicals, cleaning, and the low production rate makes the product very expensive. Finally, it is even more difficult and time consuming to produce a pattern of conductive metal on a fabric using this method.
U.S. Pat. No. 3,152,313, issued Oct. 6, 1964 to Barbour et al., discloses an elastic heater comprising of a wire heating element attached to an elastic cloth. The wire heating element is bent in a zigzag shape to allow the wire to straighten out with the stretching of the elastic cloth and return to a zigzag shape when the elastic cloth returns to its resting state. This disclosure is limited in stretch by the straight length of the heating wire in the stretched state. Further wire fatigue will become an issue over repeated use. This disclosure is used to cover an outer surface and not to cover an inner surface of a void.
U.S. Pat. No. 5,714,738 issued Feb. 3, 1998 to Hauschulz et al., discloses a heater mat that is preferably made of two layers of fiberglass reinforced rubber sheets laminated together with resistive heater wires sandwiched between the laminated sheets. The heater mat is formed with a curvature and size to fit snugly around the peripheral surface of the pipe that is to be heated. A jacket of thermally insulative material, such as a polymer foam, is molded over the external surface of the heater mat. The mat and the jacket are configured so that the heater has interfacing opposite edges that meet and preferably touch each other when the heater is mounted onto the pipe, but the combination of the mat and jacket have sufficient resilient flexibility to allow opening the heater by separating the edges enough to slip the heater over the pipe, whereupon the heater resumes its original inherent cylindrical shape when released. Snaps, Velcro™ fastening material straps, or other suitable fasteners can be used to secure the heaters snugly around the pipe, if desired, although the biased resilience of the heater to its formed shape is generally sufficient itself to hold the heater in place. The disclosure uses resistive heaters, has a fixed shape defined by a mold, and is only flexible enough to snap around an object.